The present invention relates to an improved filter for oil flow from the sump of a transmission to the oil pump supplying oil to an automatic transmission. Presently utilized oil filters consist of a fine mesh strainer or bag extending across a filter housing with a housing input from the oil sump on one side of the strainer and a housing outlet on the opposite side communicating with the suction fluid passage leading to a transmission oil pump. As air is entrained in the oil circulating through the automatic or continuously variable transmission, air bubbles tend to form along the fine mesh screen in the filter due to reduced pressure therein resulting from the suction side of the oil pump. The bubble size increases as vacuum increases and these large air bubbles pass through the screen and are released into the suction passage leading to the pump mixed with the oil.
Large air bubbles create a condition that is most critical in variable displacement vane pumps running at minimum displacement, but the condition is present in all automatic transmission systems using an enclosed filtration system. Where the large air bubbles are released and pass through the variable displacement pump under minimum displacement, such as occurs in the minimum ratio range of a continuously variable transmission or in the top ratio range of an automatic transmission, a periodic release of the hydraulically-actuated drive system is experienced causing a neutral condition due to premature hydraulic element release. The present invention overcomes this problem.